1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier, printer, facsimile device, printing press or composite machines thereof, and more particularly to an image transfer device of a type for forming an electrostatic latent image on an image carrier and performing a primary transfer of a toner image obtained by developing this electrostatic latent image onto an endless belt serving as an intermediate transfer member at a primary transfer portion, that is to say, a primary transfer nip position, and performing a secondary transfer of the toner image that has been primary transferred onto the endless belt onto a recording medium, such as a sheet, carried to a secondary transfer portion, that is to say, a secondary transfer nip position.
2. Description of the Related Art
While a seamless, endless belt is preferably employed as the intermediate transfer member in the image transfer device employed in this kind of image forming apparatus, for reasons of cost a seamed, endless belt is commonly employed. However, because the seam of a seamed, endless belt protrudes therefrom, undesirable load and torque fluctuations attributable to this protruding section are generated as the belt is moved when the seam passes the primary transfer nip position and the secondary transfer nip position. These fluctuations affect the movement of the belt and cause a change in the moving speed that leads to unstable belt movement.
On the other hand, when the front-end portion of the sheet serving as the recording medium P (particularly in the case of a thick sheet (extra thick paper)) goes into the secondary transfer nip position and the rear-end portion thereof comes out of the secondary transfer nip position, load and torque fluctuations attributable to the thickness of the recording medium P that which cause unstable belt movement occur. Because these fluctuations are transferred by way of the endless belt to the primary transfer portion, that is to say, the primary transfer nip portion, fluctuations in the correct transfer position are produced at the primary transfer nip position resulting in generation of a so-called shock jitter. Accordingly, warp leading to unsatisfactory image production is generated in the toner image transferred onto the sheet.
The fluctuations affecting the stable movement of the endless belt are attributable to several other factors apart from the protruding seam of the endless belt and the employment of a thick recording medium P such as extra thick paper. These include, for example, minute belt slip caused by load and torque fluctuations, belt stretching, deflection during rotation at the joint mechanism or gear portion serving as the decelerator mechanism of the transmission system and, furthermore, inability of the retention force of the drive motor to withstand the load and torque fluctuations.
Increasing the output of the drive sources for driving the endless belt and improving the drive transmission rigidity of the decelerator for transmitting the drive force of the drive sources have been considered as means to obviate the various problems inherent to the prior art described above. Turning of an endless belt around a tension roller urged by a spring or the like of a conventional image forming apparatus and movement of the position of the tension roller to absorb torque fluctuation so as to alleviate shock has been proposed (for example, Japanese Patent No. 3,294,342 and Japanese Laid-open Patent Application No. H06-115752).
However, concerns exist regarding the increased size and weight and so on of the apparatus and associated increase in costs that results from increasing the output of the drive force and improving the drive transmission rigidity of the decelerator. In addition, in apparatus in which the endless belt is turned around a tension roller, when the mass of the member for absorbing shock is increased a drop in responsivity and inability to withstand high-frequency sudden load fluctuation occurs and, accordingly, the desired improvement cannot be achieved.
On the other hand, image forming apparatuses that utilize a system based on integrated configuring of a section from a secondary transfer portion to a fixing portion have been proposed in recent years with a compacting of the apparatuses and improved reliability of the sheet carry thereof being achieved as a result (for example, Japanese Laid-open Patent Application Nos. H10-63121 and 2004-145260).
However, there is a problem inherent to these image forming apparatuses in that, because the load fluctuation that occurs when the front-end portion and rear-end portion of the extra thick paper serving as the recording medium P pass through the fixing nip portion is transferred by way of the intermediate transfer belt to the primary transfer nip position, the shock jitter described above which leads to generation of warp in the transfer image occurs.
Technologies relating to the present invention are disclosed in, for example:
Japanese Patent No. 3,042,414,
Japanese Patent No. 3,514,134,
Japanese Laid-open Patent Application. No. S63-189878,
Japanese Laid-open Patent Application. No. H04-360179,
Japanese Laid-open Patent Application. No. H06-308839,
Japanese Laid-open Patent Application. No. H10-268595,
Japanese Laid-open Patent Application. No. H11-231677,
Japanese Laid-open Patent Application. No. 2000-089601
Japanese Laid-open Patent Application. No. 2001-092274,
Japanese Laid-open Patent Application. No. 2001-296764,
Japanese Laid-open Patent Application. No. 2002-268325,
Japanese Laid-open Patent Application. No. 2002-278311,
Japanese Laid-open Patent Application. No. 2002-304080 and
Japanese Laid-open Patent Application. No. 2005-309227.